Commonly owned U.S. Pat. Nos. 6,677,915 filed Feb. 12, 2001, titled “SHIELDED SPIRAL SHEET ANTENNA STRUCTURE AND METHOD”; 6,906,667 filed Feb. 14, 2002, titled “MULTIFREQUENCY MAGNETIC DIPOLE ANTENNA STRUCTURES FOR VERY LOW PROFILE ANTENNA APPLICATIONS”; 6,900,773 filed November 18, 2002, titled “ACTIVE CONFIGUREABLE CAPACITIVELY LOADED MAGNETIC DIPOLE”; and 6,919,857 filed Jan. 27, 2003, titled “DIFFERENTIAL MODE CAPACITIVELY LOADED MAGNETIC DIPOLE ANTENNA”; describe an Isolated Magnetic Dipole (IMD) antenna formed by coupling one element to another in a manner that forms a capacitively loaded inductive loop, setting up a magnetic dipole mode, the entire contents of which are hereby incorporated by reference. This magnetic dipole mode provides a single resonance and forms an antenna that is efficient and well isolated from the surrounding structure. This is, in effect, a self resonant structure that is de-coupled from the local environment.
The overall structure of the IMD antenna can be considered as a capacitively loaded inductive loop. The capacitance is formed by the coupling between the two parallel conductors with the inductive loop formed by connecting the second element to ground. The length of the overlap region between the two conductors along with the separation between conductors is used to adjust the resonant frequency of the antenna. A wider bandwidth can be obtained by increasing the separation between the conductors, with an increase in overlap region used to compensate for the frequency shift that results from the increased separation.
An advantage of this type of antenna structure is the method in which the antenna is fed or excited. The impedance matching section is almost independent from the resonant portion of the antenna. This leaves great flexibility for reduced space integration. The antenna size reduction is obtained in this case by the capacitive loading that is equivalent to using a low loss, high dielectric constant material. At resonance a cylindrical current going back and forth around the loop is formed. This generates a magnetic field along the axis of the loop which is the main mechanism of radiation. The electrical field remains highly confined between the two elements. This reduces the interaction with surrounding metallic objects and is essential in obtaining high isolation.
The IMD technology is relatively new, and there is a need for improvements over currently available antenna assemblies. For example, because cell phones and other portable communications devices are moving in the direction of providing collateral services, such as GPS, video streaming, radio, and various other applications, the demand for multi-frequency and multi-band antennas is at a steady increase. Other market driven constraints on antenna design include power efficiency, low loss, reduced size and low cost. Therefore, there is a need in the art for antennas which exceed the current market driven requirements and provide multiple resonant frequencies and multiple bandwidths. Additionally, there is a need for improved antennas which are capable of being tuned over a multitude of frequencies. Furthermore, there is a need for improved antennas which are capable of dynamic tuning over a multitude of frequencies in real time.